The Fischer-Tropsch reaction involves the catalytic hydrogenation of carbon monoxide to produce a variety of products ranging in size and functionality from methane to higher alcohols. The methanation reaction was first described by Sabatier and Senderens in 1902. The later work of Fischer and Tropsch dealing with higher hydrocarbons was described in Brennstoff-Chem. 7, 97 (1926).
The reaction is highly exothermic and care must be taken to design reactors for adequate heat exchange capacity. Nevertheless, substantial research has been undertaken in the interim since the initial characterization of the reaction during the 1920's. The process is especially suitable for use when carbonaceous feedstocks of otherwise low economic value are available. For instance, the first major commercial use of the Fischer-Tropsch process was in Germany during the mid-30's. By the beginning of World War II, Germany was producing nearly 11,000 B/D of primary products using mainly the cobalt-based catalyst described by Fischer and Pichler (Ger. Pat. No. 731,295 issued Aug. 2, 1936). The feedstock was, in general, based on available coals.
Subsequently, a consortium of nine American companies designed and built a plant at Brownsville, Tex. based on an iron-based catalyst. The plant was completed in 1950 and had a design capacity of 50MMSCFD. Various economic and technical difficulties caused final shutdown of the plant in the late 50's.
A reasonably economic use of the process has been practiced in South Africa in the SASOL plants. These plants use an iron-based catalyst and produce gasoline and waxes by gasifying a somewhat low-grade coal to produce a synthesis gas for feed to the Fischer-Tropsch reactors.
Reseach continues in this area because of the potential for converting low value feedstocks into higher value products.
The chemistry of the Fischer-Tropsch reactions is, in a gross sense, quite simple. The overall reactions for the production of alkanes (No. 1), alkenes (No. 2) and alcohols (No. 3) are as follows:
1. (2n+1)H.sub.2 +nCO.fwdarw.C.sub.n H.sub.2n+2 +nH.sub.2 O PA0 2. 2nH.sub.2 +nCO.fwdarw.C.sub.n H.sub.2n +nH.sub.2 O PA0 3. 2nH.sub.2 +nCO.fwdarw.C.sub.n H.sub.2n+1 OH+(n-1)H.sub.2 O
(n+1)H.sub.2 +2nCO.fwdarw.C.sub.n H.sub.2n+2 +nCO.sub.2 PA1 nH.sub.2 +2nCO.fwdarw.C.sub.n H.sub.2n +nCO.sub.2 PA1 (n+1)H.sub.2 +(2n-1)CO.fwdarw.C.sub.n H.sub.2n+1 OH+(n-1)CO.sub.2
The types and amount of products obtained via such reactions are typically dependent upon the reaction conditions and choice of catalyst.
Few of the catalysts used in the past have been either very selective or very active. Those catalysts that were selective or active were uneconomic for other reasons, e.g., sensitivity to sulfur poisoning or used high cost catalytic metals such as ruthenium.
The catalyst of the present invention is iron/carbon-based. Because of the method of its preparation, the catalyst has high selectivity and/or conversion at reaction conditions considered to be quite moderate.
As noted above in the historical discussion, iron-bearing catalysts were among the first ever used in the Fischer-Tropsch reaction. Indeed, Fischer and Tropsch believed that carbides were an intermediate in the overall reaction. Later kinetics work suggested carbides could not be an intermediate in the process. Hall et al, J. Soc. Chem. Ind. London 65, 128 (1946); Weller, J. Am. Chem. Soc. 69, 2432 (1947) and; Kummer et al, J. Am. Chem. Soc. 70, 3632 (1948). However, the reduced metallic iron, as used in the Lurgi-Ruhrchemie fixed bed process, appears to change from the original .alpha.-Fe phase to a mixture of .alpha.-Fe, Fe.sub.3 O.sub.4, FeC and Fe.sub.2 C as conversion operations continue. See, Malan et al Brennstoff-Chem. 42, 209-212 (1961).
The present invention, as will be discussed below in greater detail, involves the use of a laser to pyrolize low valence iron-carbon bearing compounds to produce a fine particle iron-carbon containing catalyst. At least a portion of the catalyst is the iron carbide, cementite.
Others have described the use of iron-carbon containing catalysts produced by laser pyrolysis in Fischer-Tropsch reactions. The work of Gupta et al (in U.S. Pat. No. 4,468,474), issued Aug. 28, 1984 and in SPIE 458, Appl. of Lasers to Industrial Chemistry, 131-139 (1984)) shows the production of iron, carbon and silicon-containing catalysts by a laser and the catalysts' subsequent use in the Fischer-Tropsch process. Moderate activity and high C.sub.2 -C.sub.4 olefin selectivity is asserted for the catalysts.
Applicants' catalysts contain substantially no silicon.
No known prior art is believed to show the use of the catalyst described below in the efficient production of heavy hydrocarbons.